Major improvement on detection of contaminating particles in solar cell production

13 March 2015 by Communication TNW

Detection and localization of isolated contaminations on a flat surface becomes increasingly difficult when they become smaller than can be resolved by cameras (below the diffraction limit for visual illumination).

The possible solutions for this problem are quite complex and often not usable for very fast operations or out-of-lab implementations.
Researchers at the Applied Sciences Optics Research Group have now developed a simple and robust configuration similar to standard scanning confocal microscopes, to achieve this goal. In this configuration, instead of a camera, only a pair of differential detectors are needed. The asymmetry in power distribution among the halves of the detector is used as signal.

The technique is further benefitted by using radially polarized light which allows a coarser sampling and hence, a faster operational speed. The scientists (with PhD-student Sarathi Roy as leading author) have published their results in Physcial Review Letters (March 13th  issue, ‘Radially polarized light for detection and nanolocalization of dielectric particles on a planar substrate’).

The new method is a major improvement on an earlier development by the Optics Research Group (as part of the larger European project Clean4Yield). Last year they successfully installed a scatterometer at the Holst Centre in Eindhoven, the research centre of TNO and IMEC. The system is suitable for high-speed inspection of small contamination or damages in production environment, which, till now, is a bottleneck for high volume production in several technologies, such as roll to roll production of organic electronic devices, like solar cells.

The new method is much faster and more accurate. Not only are the small particles detected, they are also localized. The key to this is the use of radially polarized light instead of linear polarized light, which makes the set-up very suitable for the use of two differential detectors. 

The improved scatterometer in Eindhoven can be used for the production of solar cells and organic LEDs (OLEDs) from thin plastic film. In principle, this process is extremely cheap but a  major problem is the detection and removal of small, contaminating particles. This primarily involves particles with a diameter of between 100 and 500 nanometres.

The improved Optics Research Group’s scatterometer now offers a potential solution for these smaller particles. The removal of small particles could be a major step forward towards the production of cheaper solar cells and OLEDs.

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